1. Field of the Invention
The present invention relates to a sensitive body, for electronic photograph use, using amorphous silicon, and, more particularly, to a photoreceptor for electrophotography utilizing a-Si:N:H:F.
2. Description of the Prior Art
Materials such as amorphous selenium (a-Se) or compositions of resin with cadmium sulfide being dispersed therein have been most commonly used as a photoreceptor for electrophotography. However, the former photoreceptor of amorphous selenium is vulnerable to heat or mechanical impacts so that the surfaces thereof might be easily broken or deteriorate and thereby lose sensitivity. Also, both the photoreceptors of amorphous selenium and cadmium sulfide are not mechanically solid and do not have a long service life. In addition, the materials such as being Se, Cd are well known as poisonous and harmful to the health.
Recently amorphous silicon (a-Si) has drawn attention as a material for an ideal photoreceptor which can eliminate the above disadvantages of the conventional ones, since it is considered to be highly sensitive, extremely solid, and pollution free. Particularly, a-Si:H whose unsaturated chemical bond, so called dangling bond, is terminated with hydrogen, can be doped into, a p or n type semiconductor, so that various superior electric characteristics are provided. Therefore, the a-Si is used not only as a photoreceptor, but also as a material for the other electronic components.
Some excellent characteristics can be expected from the a-Si whose dangling bond is terminated with hydrogen described hereinabove, but it can not be put into practical use, because the bond between hydrogen and silicon is so weak that some of the hydrogen is evolved by irradiation of light and heat, thus resulting in deteriorated characteristics for the material.
An amorphous silicon:H:F layer is proposed, to which fluorine has been added beside hydrogen for termination of the dangling bond. In an experiment, it is confirmed that the thin film is more stable than amorphous silicon :H while retaining almost the same optical sensitivity.
However the a-Si:H:F layer can not acquire high resistivity in the order of 10.sup.13 .OMEGA.cm which, at present, is necessary for a photoreceptor of electrophotography. Also, the sufficient resistivity thereof can not be acquired even if compensated for by the addition of boron such dopants.
On the other hand, it has recently been proposed to provide the structure of a photoreceptor consisting of amorphous silicon with an electric blocking layer of a-SiN which is inserted between the a-Si:H layer and a conductive substrate in order to retain the surface potential. Also, the material of a-Si.sub.1-x N.sub.x :H, wherein x is rather small, with a small amount of boron, is also proposed to be utilized as photosensitive layer. When these photoreceptors described hereinabove were tested in the conventional electrophotographic process in laboratory, the initial characteristics of the photoreceptor was satisfactorily attained, but it was found that the surface charging potential was reduced to half after about two to three weeks.
Such deterioration of the surface charging potential was observed almost equal to that of the boron added a-Si:H photoreceptor which was not passivated by the nitrogen. These observations should be understood as the result of the evolution of hydrogen which was put into the material to terminate the dangling bond. In the case of the a-SiN, it is considered that nitrogen has a tendency to form a nitrogen silicon compound, instead of a mere terminator, so that the dangling bond caused due to the amorphous state is mainly terminated by hydrogen. However, as described hereinabove, the termination of the dangling bond by the hydrogen is insufficient and the aging deterioration thereof cannot be avoided.